The present invention relates to a method and a device for detecting air turbulence in the environment of an aircraft, in particular of a transport plane.
More particularly, although not exclusively, the present invention is aimed at detecting and predicting, in the short term, turbulence such as clear air turbulence of CAT type or orographic waves in particular.
It is known that wind shears, that is to say abrupt variations in the strength and/or direction of the wind, are sometimes triggered with no visible manifestation, (generally above 15,000 feet, about 4500 meters) and are, therefore, called clear air turbulence (or clear sky turbulence). Such turbulence which is non convective generally appears in a hydrostatically stable air environment, that is to say in a region where the potential temperature (which is dependent on the static temperature and on the static pressure) will increase with altitude. Clear air turbulence usually appears near the tropopause, especially above mountains and especially in winter, and in particular in the vicinity of jet streams.
Such turbulence cannot generally be detected beforehand by way of standard airborne weather radars. Moreover, such clear air turbulence may last several minutes and bring about, in particular, the following effects:                variations in vertical acceleration which, when they are negative, are liable to injure passengers;        changes in the flight level (altitude), thereby increasing the risk of collision with the terrain overflown or with other aircraft; and        a contribution to the fatigue of the airfoil        
Such turbulence therefore exhibits an unpredictable nature which may surprise the crew and the onboard systems. Clear air turbulence presents a natural risk for aircraft in flight. An aircraft in equilibrium may therefore be disturbed by passing through such a zone of turbulence. Moreover, turbulence of this type gives rise to additional fuel consumption (in particular on account of the necessity to avoid such a turbulence zone) and contributes to the fatigue of the aircraft and pilots (stress).
It is known that today weather radars allow the aircraft to distinguish precipitation zones to be avoided, generally within a radius of 100 nautical miles (about 180 kilometers). A display is usually provided on a navigation page which allows the pilot to judge the safest route to follow. Phenomena such as convective turbulence may be displayed within a radius of 40 nautical miles (about 75 kilometers), and air shear phenomena within a radius of 10 nautical miles (about 18 kilometers). However, weather radars are limited to the detection of convective turbulence exhibiting sufficient humidity, since such radars carry out Doppler-effect-based detection of the relative movement of humid air masses. They are therefore not operational below a moderate level of reflectivity.